The invention relates to a method for operating an exhaust gas emission control system of a motor vehicle internal combustion engine, in which an oxidation-catalytically active exhaust gas emission control component exhaust is arranged in the gas line upstream of an SCR catalyst, wherein an ageing state of the oxidation-catalytically active exhaust gas emission control component is determined.
The use of an oxidation-catalytically active exhaust gas emission control component in exhaust gas emission control systems of motor vehicles with an internal combustion engine is common for reducing damaging exhaust gas emissions. The knowledge of the ageing state of the oxidation-catalytically active exhaust gas emission control component is particularly important with the presence of further exhaust gas emission control systems connected downstream. With a decrease of the catalytic cleaning capability of the oxidation-catalytically active exhaust gas emission control component caused by ageing, the operation of the exhaust gas emission control system can be adapted correspondingly or a necessary change can be indicated.
DE 197 32 167 A1 discloses a method for determining the ageing state of an oxidation-catalytically active exhaust gas emission control component where an exhaust gas emission control characteristic of the exhaust gas emission control component is determined and compared to a reference characteristic. In particular, an ageing is diagnosed if impermissible deviations from the reference characteristic result in an ascent section.
With this method, the necessity of having to provide a corresponding sensor system for determining the exhaust gas emission control characteristics is disadvantageous.
Exemplary embodiments of the present invention provide a method for operating an exhaust gas emission control system with an oxidation-catalytically active exhaust gas emission control component, in which an ageing state of the oxidation-catalytically active exhaust gas emission control component can be determined with little effort regarding the apparatus.
Exemplary embodiments of the present invention provide a method for operating an exhaust gas emission control system of a motor vehicle internal combustion engine, in the exhaust gas line of which an oxidation-catalytically active exhaust gas emission control component is arranged upstream of a SCR-catalyst in which an ageing state of the oxidation-catalytically active exhaust gas emission control component is determined by correlating a hydrocarbon fraction present in the exhaust gas emission upstream of the oxidation-catalytically active exhaust emission component with a simultaneous nitrogen oxide conversion of the SCR-catalyst.
The ageing state of the oxidation-catalytically active exhaust emission control component is determined with the method of the invention by correlating a hydrocarbon fraction present in the exhaust gas upstream of the oxidation-catalytically active exhaust gas emission component with a simultaneously present nitrogen oxide conversion of the SCR-catalyst.
The invention uses the surprising finding that a conversion of nitrogen oxides (NOx) to nitrogen (N2) that can be achieved by the SCR catalyst depends on the ageing state of the oxidation-catalytically active exhaust gas emission control component connected upstream at least with certain operating conditions. This is due to the ability of the oxidation-catalytically active exhaust gas emission control component to be able to change the exhaust gas composition by oxidation of oxidizable exhaust gas components such as nitrogen monoxide (NO), hydrogen (H2), carbon monoxide (CO) and/or hydrocarbons (HC). A change of the exhaust gas component, particularly an increase of the fraction of nitrogen dioxide (NO2) effected by the oxidation-catalytically active exhaust gas emission control component has again a repercussion on the achievable NOx conversion of the SCR catalyst. The ability to change the exhaust gas composition is, however, reduced with an increasing ageing of the oxidation-catalytically active exhaust gas emission control component. According to the invention, HC is consulted as a test substance for checking the capability of the oxidation-catalytically active exhaust gas emission control component for changing the exhaust gas composition and thus for influencing the NOx conversion capability of the SCR catalyst, and a determination of the ageing state of the oxidation-catalytically active exhaust gas emission control component takes place by a correlation of the HC fraction effective upstream of the oxidation-catalytically active exhaust gas emission control component with the simultaneously present NOx conversion of the SCR catalyst.
Generally, an oxidation-catalytically active exhaust gas emission control component is meant to be each component serving for an exhaust gas cleaning, which supports at least an oxidation of NO. The oxidation-catalytically active exhaust gas emission control component can be, for example, an oxidation catalyst or a particle filter with an oxidation-catalytically active coating or a combination of the mentioned components. An SCR catalyst is a catalyst that can catalyze a reduction of NOx under oxidizing conditions, particularly by means of ammonia (NH3) as selective reducing agent. It is typically in particular an iron-containing zeolitic SCR catalyst.
In an arrangement of the method according to the invention, different HC fractions are adjusted in the exhaust gas upstream of the oxidation-catalytically active exhaust gas emission control component and a HC fraction associated with an undercutting of a presettable NOx conversion is consulted as the basis for determining the ageing state. By a variation of the HC fraction, it can be determined in a more exact manner if and how far the HC fraction upstream of the oxidation-catalytically active exhaust gas emission control component influences a change of the exhaust gas composition, and in particular the NO2 content downstream of the oxidation-catalytically active exhaust gas emission control component and to what extent the NOx conversion of the SCR catalyst is influenced. By the correlation of the NOx conversion and the HC content, an ageing state can be concluded. A comparison with reference values can be carried out, which were previously determined empirically and which are, for example, available as ageing characteristic line. The HC can be present in the exhaust gas as non-combusted or partially combusted fuel components and can, for example, be introduced into the exhaust gas by motoric after-injection and/or by a secondary injection of fuel into the exhaust gas effected from the outside. It could be determined that the amount of the HC content, where a typically present NOx maximum conversion of the SCR catalyst is undercut, represents a reliable measure for the ageing state of the oxidation-catalytically active exhaust gas emission control component and thus characterizes this in a particularly reliable manner.
In a further arrangement of the invention, the adjustment of different HC fractions in the exhaust gas takes place at an operating point with a temperature of the oxidation-catalytically active exhaust gas emission control component, where an oxidation of NO to NO2 catalyzed by the oxidation-catalytically active exhaust gas emission control component. The operating point is characterized by temperatures of about 200° C. to 350° C., chamber speeds of the exhaust gas of about 50000 1/h and oxygen contents in the exhaust gas of 2% to 15%. A reduced HC conversion of the oxidation-catalytically active exhaust gas emission control component caused by ageing strengthens an inhibition of the NO2 formation typically present effected by HC, which is particularly noticeable at an operating point where the NO2 formation of an oxidation-catalytically active exhaust gas emission control component that has not aged is approximately maximal. With a reduced NO2 formation the NOx conversion of the SCR catalyst arranged downstream is, however, typically reduced. This is why the ageing state of the oxidation-catalytically active exhaust gas emission control component can be determined in a particularly reliable manner by correlation of the HC content and NOx conversion by means of the method procedure of the invention.
The adjustment of different HC fractions in the exhaust gas takes place in a further arrangement of the invention at an operating point with a temperature of the SCR catalyst in the region between 180° C. and 350° C. A comparatively strong dependence for the NOx conversion capability of the SCR catalyst on the NO2 content or from the ratio of NO2 to NOx of the exhaust gas is given in this temperature region, wherein the oxidation-catalytically active exhaust gas emission control component can oxidize NO to NO2 on the other hand in dependence on age and in dependence on the HC content of the exhaust gas in a more or less effective manner. In the region between 180° C. and 350° C., and in particular at about 200° C., a particularly reliable determination of the ageing state is enabled by correlation of HC content and NOx conversion for conventional SCR catalysts. An operating point with an exhaust gas chamber speed of about 50000 1/h with regard to the SCR catalyst is preferred.
In a further arrangement, an adaption of a characteristic field is provided for the operation of the exhaust gas emission control system for an oxidation of NO to NO2 effected by the oxidation-catalytically active exhaust gas emission control component depending on the determined ageing state. Characteristic fields for the NO2 formation are typically used for a modeling of a NOx conversion of the SCR catalyst or for the model-based dosing of the reducing agent used at the SCR catalyst for a NOx conversion. If a particle filter is provided in the exhaust gas line, its soot load state can be modeled depending on a NO2 concentration present in the exhaust gas. Points of times are fixed in a fitted manner thereto, where a thermal forced regeneration is to be carried out. By means of an adaptation or adaption of a provided characteristic field or a characteristic line regarding the NO2 formation dependent on the operating state to the ageing of the oxidation-catalytically active exhaust gas emission control component, the mentioned and possibly further operating variable can be fixed optimally over the operation duration of the exhaust gas emission control system.
In this manner, an excessive deterioration of the cleaning effect of the exhaust cleaning system altogether caused by the ageing of the oxidation-catalytically effective exhaust gas emission control component and an undesired higher fuel usage due to more frequent forced generations of the particle filter are avoided.
Advantageous embodiments of the invention are illustrated in the drawings and are described in the following. The previously mentioned characteristics and which will still be explained in the following cannot only be used in the respectively given combination but also in other combinations or on their own without leaving the scope of the present invention.